Topological insulators are insulators in the bulk but metals on the surface, and the electrons that flow swiftly across their surfaces are “spin polarized.” Surface-electron spin and momentum are locked, offering new ways to control electron flow and distribution in spintronic devices. A Nature Physics paper by first author Chris Jozwiak of the Advanced Light Source and a large team led by Alessandra Lanzara and Zahid Hussain describes surprising results counter to previous assumptions: the spin polarization of photoemitted electrons from the surface of a topological insulator is wholly determined in three dimensions by the polarization of the incident light beam.

The diagram at right shows the electronic states of bismuth selenide in momentum space. ARPES, at left, can directly create such maps with photoelectrons. A slice through the conduction cone at the Fermi energy maps the topological insulator’s surface as a circle (upper left); here electron spins and momenta are locked together. Initial ARPES measurements in this experiment were made with p-polarized incident light in the regions indicated by the green circle and line, where the spin polarization of the photoelectrons is consistent with the intrinsic spin polarization of the surface.